As electronics grow smaller while expanding in functionality, the demand for higher energy density is increasing. For portable electronics, current batteries are failing to meet this demand. The ever-growing demand for wireless sensors and portable electronics has spurred research of new power sources to enhance system performance and extend lifespan.
New technologies such as microscale heat engines, micro fuel cells, micro thermo-photovoltaic and micro-thermoelectric generation are being developed as possible high energy-density alternatives to traditional batteries. Additionally, there is great interest for environmental energy harvesting to locally generate power for distributed sensor networks or wireless devices that never require battery charging or replacement.
Recently, approaches for addressing power needs have become directed to converting waste-heat from a device into energy for that device. This can be accomplished through the thermoelectric effect, which is the direct conversion of temperature differences to electric potential. Thermoelectric (TE) devices have been the popular choice for direct energy conversion between thermal and electrical domains for a number of years. They find widespread application owing to their advantages such as absence of moving parts, ease of fabrication, robustness, and reliability.
The typical TE module utilizes a parallel plate structure where a heated surface is in direct contact with one plate while the other plate is kept cool. This has been implemented for microscale applications using bulk p/n type semiconductors. An example of a typical parallel plate TE module is shown in FIG. 1. A thermocouple 20 (shown in the breakout image of FIG. 1) is formed from two dissimilar conducting materials joined at one end. One of the conducting materials can be p-type semiconductor material and the other of the conducting materials can be n-type semiconductor material. These two couples can be joined at one end using a metal. A voltage (Voc) is developed across the open contacts when a temperature difference (THot−TCold) is sustained along the two couples.
Referring to FIG. 1, a typical macroscale TE module includes an array of such thermocouples 20 sandwiched between two rigid plates 21 and 22. As described above, each thermocouple pair includes two dissimilar conductors (Leg1 and Leg2), often p- and n-type semiconductors. In the array, the thermocouples 20 are connected electrically in series through metal interconnect 23 and thermally in parallel to form the thermopile. For power generation, a heated surface is brought into contact with one of the plates 21, while the other plate 22 is kept cool. The resulting temperature gradient across the thermopile generates a voltage potential (Voc) across the open ends due to the Seebeck effect. When connected to a load, current will flow, and electrical power is supplied by the generation of the voltage potential caused by the temperature gradient.
Thermoelectric materials often used for TE devices are selected based on their heat transformation characteristics and current flow direction. Accordingly, material development is being conducted to provide high Figures of Merit (ZT).
Recent advancements in thin-film semiconductor alloys with high thermoelectric Figures of Merit (ZT) have attracted great interest in TE devices, especially for small-scale applications. Despite these advances, the integration of these thin-film materials into miniaturized generator platforms remains technologically challenging. Several efforts have focused on integration of semiconductor TE materials on micromachined silicon platforms with the goal of achieving smaller, lighter, high-power density, high-efficiency cooling or power generation systems. To achieve a functional TE module, one major challenge is the integration and reliable interconnection of numerous series-connected thermoelements, especially if two or more substrates are bonded together. Another challenge is maintaining high thermal gradients in physically small structures. Furthermore, present challenges to providing efficient thermoelectric power generation include thermal leakage and the temperature difference needed across hundreds of micrometers.